In the electrophotographic screening (EPS) process described in U.S. Pat. Nos. 4,921,767, issued to Datta et al., on May 1, 1990 and in 5,229,234, issued to Riddle et al. on Jul. 20, 1993, dry-powdered, triboelectrically charged, color-emitting phosphors are deposited, serially, on an electrostatically chargeable photoreceptor having a dry-powdered, triboelectrically charged, light-absorbing matrix thereon. The photoreceptor comprises an organic photoconductive (OPC) layer overlying, preferably, an organic conductive (OC) layer, both of which are deposited, serially, on an interior surface of a CRT faceplate panel. Initially, the OPC layer of the photoreceptor is electrostatically charged to a positive potential, using a suitable corona discharge apparatus of the type described in U.S. Pat. No. 5,083,959, issued to Datta et al. on Jan. 28, 1992. Then, selected areas of the photoreceptor are exposed to visible light to discharge those areas, without affecting the charge on the unexposed area. Next, triboelectrically negatively charged, light-absorbing material is deposited, by direct development, onto the charged, unexposed area of the photoreceptor to form a substantially continuous pattern of light-absorbing material, hereinafter called a matrix, having open areas therein. In order to achieve sufficient optical density, or opacity, of the EPS-deposited matrix, it is necessary to build-up a sufficient amount of light-absorbing material. This, however, results in a matrix having a relatively rough surface. The photoreceptor and the matrix are recharged by the corona discharged apparatus to impart an electrostatic charge thereon. It is desirable that the charge on the photoreceptor be of the same magnitude as that on the previously deposited matrix; however, it has been determined that the photoreceptor and the matrix do not necessarily charge to the same potential. In fact, the charge acceptance of the matrix is different from the charge acceptance of the photoreceptor. Consequently, when different selected areas of the photoreceptor are exposed to visible light to discharge those areas, to facilitate reversal development with triboelectrically positively charged color-emitting phosphor materials, the matrix retains a positive charge of a different magnitude than the positive charge on the unexposed area of the photoreceptor. This charge difference influences the deposition of the positively charged color-emitting phosphor materials, causing the phosphors to be more strongly repelled by the charge on the matrix, than by the charge on the unexposed area of the photoreceptor. This stronger repelling effect of the matrix causes the color-emitting phosphors to be slightly displaced from their desired locations on the photoreceptor. The repelling effect of the matrix is small, nevertheless, the effect is sufficient to narrow the width of the color-emitting phosphor lines so that the lines do not contact and overlap the edges of the matrix. Thus, slight gaps occur between the phosphor lines and the surrounding matrix. These gaps are unacceptable because they reduce the brightness of the phosphor in each picture element. Furthermore, the gaps are visible when the screen assembly is aluminized to provide a reflective backing and anode contact to the screen assembly.
One method of reducing the repulsive effect of the EPS-deposited matrix is described in co-pending U.S. patent application Ser. No. 250,231, filed on May 27, 1994 by Ritt et al., and entitled, METHOD OF ELECTROPHOTOGRAPHIC PHOSPHOR DEPOSITION. In that application, rather than using an EPS-deposited matrix, a conventional wet slurry matrix is formed by the process described in U.S. Pat. No. 3,558,310, issued to Mayaud on Jan. 26, 1971. The conventional matrix is formed directly on the interior surface of the faceplate. The conventional matrix is thin and smooth, and has the desired opacity so that the OC and OPC layers can be deposited directly thereon. Additionally, the overlying OC and OPC layers eliminate the electrostatic interaction between the matrix and the EPS-deposited phosphors. However, to improve the efficiency of the screening operation, and to have an entirely dry screening process, it is desirable also to deposit the matrix by the EPS process, but without the above-described deleterious electrostatic interaction.
A need thus exists to electrically isolate the prior EPS-deposited matrix so that the matrix is not electrostatically charged during the EPS deposition of the three color-emitting phosphors, and to form a planarizing layer that provides a smooth surface for the subsequent processing of the screen assembly, so that the phosphors are properly registered with respect to the matrix.